There are over 2.4 million breast cancer survivors (BCS) in the U.S. accounting for 22% of all cancer survivors [1]. The 5-year survival rate of 88.6% [1] coupled with a 1 in 8 lifetime chance of a breast cancer diagnosis [2] has resulted in an increasingly growing population with distinct public health needs resulting from side effects associated with current available treatments [3]. In addition, BCS are at an increased risk of cancer recurrence, comborbidites, premature death and decreased quality of life (QOL) [4-6]. Physical activity (PA) has been identified as a lifestyle factor with excellent potential for ameliorating the aversive side effects of treatment and positively influencing disease-related outcomes [7, 8]. However, only 1/3 of BCS meet the current public health guidelines for PA [9, 10]. Identifying factors influencing PA participation and how PA is related to QOL in BCS is important for developing effective interventions which can be translated into practice. This project will adopt a prospective design to test the full Social Cognitive Theory (SCT) model for explaining PA in BCS over the course of a 6-month period. Previous research has used individual elements of SCT to understand and predict PA behavior in the general population [11-13], but the full model has yet to be tested, particularly in BCS. The proposed study will include measures to test the full SCT: self-efficacy, goals, outcome expectations, sociocultural factors (facilitators and impediments), and PA behavior [14]. PA has also been shown to positively influence QOL in BCS [7, 8]. However, very little research has been conducted to explain this relationship. Thus, the proposed study will also include measures to test the PA and QOL model proposed by McAuley and colleagues [15] in BCS. This model proposes PA influences global QOL indirectly through its effects on self-efficacy and health related QOL (physical and mental health) whereby self-efficacy is both directly and indirectly related to global QOL through its effect on health related QOL. The public health significance of this study lies in its potential to develop an improved, theoretically sound understanding of PA behavior in BCS as well as a better understanding of the relationship between PA and QOL. These findings could potentially result in better designed, more effective PA programs for BCS which ultimately has the potential to reduce healthcare costs associated with cancer recurrence, comorbidities, and premature death in BCS.